The present inventions generally relate to microwave filters, and more particularly, to microwave filters designed for narrow-band applications.
Filters have long been used in the processing of electrical signals. For example, in communications applications, such as microwave applications, it is desirable to filter out the smallest possible passband and thereby enable dividing a fixed frequency spectrum into the largest possible number of bands.
Such filters are of particular importance in the telecommunications field (microwave band). As more users desire to use the microwave band, the use of narrow-band filters will increase the actual number of users able to fit in a fixed spectrum. Of most particular importance is the frequency range from approximately 800-2,200 MHz. In the United States, the 800-900 MHz range is used for analog cellular communications. Personal communication services are used for the 1,800 to 2,200 MHz range.
Historically, filters have been fabricated using normal, that is, non-superconducting materials. These materials have inherent lossiness, and as a result, the circuits formed from them having varying degrees of loss. For resonant circuits, the loss is particularly critical. The quality factor (Q) of a device is a measure of its power dissipation or lossiness. Resonant circuits fabricated from normal metals in a microstrip or stripline configuration have Q""s at best on the order of four hundred. See, e.g., F. J. Winters, et al., xe2x80x9cHigh Dielectric Constant Strip Line Band Pass Filters,xe2x80x9d IEEE Transactions On Microwave. Theory and Techniques, Vol. 39, No. 12, December 1991, pp. 2182-87.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from high temperature superconductor (HTSC) materials. The microwave properties of HTSC""s have improved substantially since their discovery. Epitaxial superconductive thin films are now routinely formed and commercially available. See, e.g., R. Hammond et al., xe2x80x9cEpitaxial Tl2 Ba2Ca1Cu2O8 Thin Films With Low 9.6 GHz Surface Resistance at High Power and Above 77xc2x0 K,xe2x80x9d Applied Physics Letters, Vol. 57, pp. 825-27 (1990). Various filter structures and resonators have been formed from HTSC""s. Other discrete circuits for filters in the microwave region have been described. See, e.g., S. H. Talisa, et al., xe2x80x9cLow- and High-Temperature Superconducting Micro-wave filters,xe2x80x9d IEEE Transactions on Microwave Theory and Techniques, Vol. 39, No. 9, September 1991, pp. 1448-1554, and xe2x80x9cHigh Temperature Superconductor Staggered Resonator Array Bandpass Filter,xe2x80x9d U.S. Pat. No. 5,616,538.
Currently, there are numerous applications where microstrip narrow-band filters that are as small as possible are desired. One such application involves the use of dual-mode filters (DMF""s), which generate two orthogonal modes that occur at the resonant frequency. DMF""s include patch dual-mode microstrip patterned structures, like circles and squares. These structures, however, take up a relatively large area on the substrate. More compact dual-mode microstrip ring structures, which occupy a smaller area on the substrate than do patch structures, have been designed.
For example, FIG. 1 shows a two-pole dual-mode filter structure 40, which includes an electrically conductive meander loop resonator 42 and a dielectric substrate 44 on which the resonator 42 is disposed. The resonator 42 includes a resonator line 46 that is formed into a loop that has a square envelope. The resonator line 46 is routed, such that it forms four arms 48, each with a single meander 50. The filter structure 40 further includes orthogonal ports 52 and 54, which are used to couple to the resonator 42. The filter structure 40 also includes a small patch 56, which is attached to an inner corner of one of the meanders 50 for perturbing the electric field pattern. As a result, a pair of degenerative modes will be coupled when either of the ports 52 and 54 is excited. The degree of coupling will depend on the size of the patch 56. Without the patch 56, no perturbation will result, and thus only the single mode will be excited. In this case, when the port 52 is used, only one of the degenerate modes will be excited, and when the other port 54 is used, the field pattern is rotated 90xc2x0 for the associated degenerate mode. As illustrated, the resonator 42 generally exhibits four-quadrant symmetry to maintain orthogonality between the two degenerative modes. See J. S. Hong, xe2x80x9cMicrostrip Bandpass Filter Using Degenerate Modes of a Novel Meander Loop Resonator,xe2x80x9d IEEE Microwave and Guided Wave Letters, vol. 5, no. 11, pp. 371-372, November 1995.
As another example, FIG. 2 shows a two-pole dual-mode filter structure 60, which includes an electrically conductive meander loop resonator 62 and a dielectric substrate 64 on which the resonator 62 is disposed. The resonator 62 includes a resonator line 66 that is formed into a loop with a square envelope. The resonator line 66 is routed, such that it forms four arms 68, each with three meanders 70. The filter structure 60 further includes orthogonal fork-shaped coupling structures 72 and 74, which are distributed between the arms 68 and meanders 70. The filter structure 60 also includes a patch 76, which is attached to the inner corner of one of the meanders 70 to effect the dual-mode coupling as previously described in the filter structure 40 of FIG. 1. See, e.g., Z. M. Hejazi, xe2x80x9cCompact Dual-Mode Filters for HTS Satellite Communication System,xe2x80x9d IEEE Microwave and Guided Wave Letters, vol. 8, no. 8, pp. 1113-1117, June 2001.
As still another example, FIG. 3 shows two-pole dual-mode filter structure 80, which includes an electrically conductive meander loop resonator 82 and a dielectric substrate 84 on which the resonator 82 is disposed. The resonator 82 is similar to the resonator 62 shown in FIG. 2, with exception that it includes a resonator line 86 that is routed, such that it forms four arms 88, each with five meanders 90. The filter-structure 80 further includes orthogonal fork-shaped coupling structures 92 and 94, which are distributed between the arms 88 and meanders 90. The filter structure 80 also includes a patch 96, which is attached to the inner corner of one of the meanders 90 to effect the dual-mode coupling as previously described in the filter structure 40 of FIG. 1. See, e.g., Z. M. Hejazi, xe2x80x9cCompact Dual-Mode Filters for HTS Satellite Communication System,xe2x80x9d IEEE Microwave and Guided Wave Letters, vol. 8, no. 8, pp. 1113-1117, June 2001.
At lower frequencies, however, even these ring structures can become quite large, since resonance occurs when the ring is approximately a full electrical wavelength long. In addition, these ring structures do not necessarily address the problems associated with parasitic coupling, which becomes more prevalent as circuits are squeezed into smaller spaces. When coupling multiple resonators to make more complex narrow-band filters, the area required to accommodate the filter can grow undesirably large in order to minimize unwanted parasitic coupling between resonators and to test the package. This is particularly an issue for narrow bandwidth filters, where the desired coupling between resonators is very small, making the spacing between resonators greater. Thus, the overall size of the filter becomes even larger. For very high Q structures, like thin film HTS, significant Q degradation can occur due to the normal metal housing.
Another issue that arises in the design of narrow-band filter structures is the ability to accurately model these structures in the presence of unknown parameters, such as parasitic coupling and the introduction of mode exciting perturbations within the electrical field. In addition, computer models often use ideal capacitors to model the external capacitive coupling of dual-mode microstrip resonators. Because of the parasitic nature of physical capacitors, low quality, and effects of mounting, however, they often become undesirable when fabricating state-of-the-art HTS microstrip circuits. In order to eliminate the physical capacitors, the computer capacitor models are often replaced by distributed structures (i.e., by using the coupling between a length of the resonator and an input/output line running parallel to it). This replacement usually introduces degradation in frequency response, which is most noticeable in the shape and depth of the transmission zeros and poor alignment of the filter poles. This adverse effect can be seen in FIGS. 4 and 5, which plot the measured (dashed lines) and computed (solid lines) of the frequency responses for the resonators 60 and 80 illustrated in FIGS. 2 and 3. As shown, the transmission zeros are not well-defined, at least in part, because the coupling structures used to couple to these resonators act as distributed or quasi-distributed structures.
The present inventions are directed to novel dual-mode resonating filter structures. The filter structures contemplated by the present inventions may be planar structures, such as microstrip, stripline and suspended stripline. In preferred embodiments, the resonators may be composed of HTSC material. The broadest aspects of the invention, however, should not be limited to HTSC material, and contemplate the use of non-HTSC material as well.
The dual-mode resonator contemplated by the present inventions comprises a dielectric substrate having a region divided into four quadrants, and a resonator line forming quadrangularly symmetrical configurations within the four quadrants of the region. In this manner, the orthogonality of the degenerative modes is maintained. In preferred embodiments, the resonator line has a nominal length of one full-wavelength at the resonant frequency, and forms an outer envelope in the form of a square. Input and output couplings are used to couple to the resonator line, e.g., in a quadrangularly asymmetrical manner. In this manner, the orthogonal degenerative modes are excited without the use of electrical field perturbing patches.
The dual-mode resonators of the present inventions can be used as building blocks for a more complex filter structure. This complex filter structure comprises a dielectric substrate having a plurality of regions, each of which is divided into four quadrants, and a plurality of the resonators associated with the plurality of regions in the manner described above. In the preferred embodiment, an input coupling is coupled to a first one of the plurality of resonators, and an output coupling coupled to the last one of the plurality of resonators. One or more couplings can be used to interconnect the plurality of resonators.
In accordance with a first aspect of the present inventions, the quadrangularly symmetrical configurations are formed from four folded sections of the ring resonator line. The quadrangularly symmetrical configurations can be any one of a variety of configurations, e.g., a unidirectional bending configuration, spiraled configuration, or a meandering configuration. These configurations can be either rectilinear or curvilinear.
Although the present inventions should not necessarily be limited to this, these symmetrical configurations provide for a more compact structure. In addition, the electrical currents within parallel line segments of each folded section are in opposite directions. As a result, the far-field radiation is minimized, thereby allowing for tighter packing of multiple resonators and minimum performance degradation due to the tighter packaging. The minimized far-field radiation also limits the amount of energy coupled to lossy test packages thereby resulting in minimal impact to the resonator quality factor.
In accordance with a second aspect of the present inventions, each of the quadrangularly symmetrical configurations is symmetrical about an imaginary line and comprises a plurality of meanders (e.g., four, six, or more meanders) and a plurality of interconnecting segments. Each of the interconnecting segments on one side of the imaginary line is parallel to and opposes an interconnecting segment on another side of the imaginary line.
Although the present inventions should not necessarily be limited to this, the meandered configurations provide for a more compact structure. In addition, the electrical currents within parallel line segments of each meander, as well as the electrical currents within opposing interconnecting segments, are in opposite directions. As a result, the far-field radiation is minimized, thereby allowing for tighter packing of multiple resonators and minimum performance degradation due to the tighter packaging.
In accordance with a third aspect of the present inventions, input and output couplings are coupled to the resonator line, wherein one or both of the input and output couplings comprises a capacitor (e.g., an interdigitated, parallel plate, or discrete capacitor) that is coupled to the resonator line through a transmission line. The transmission line is directly connected to the resonator line to provide a point of contact with the resonator line. The input or output coupling can also have another transmission line for coupling to external circuitry. By way of non-limiting example, the first transmission line can be a narrow high impedance line, and the second transmission line can be a broad low impedance (e.g., 50 ohm) line connected to the external circuitry. Although the present inventions should not necessarily be limited by this, the direct coupling of the capacitor to the resonator line more accurately represent ideal lumped element capacitor connections from the computer modeling than do distributed coupling structures. If the filter structure comprises a plurality of resonator lines, one or more couplings can interconnect the plurality of resonator lines. Each of these interconnecting couplings can include a common coupling segment, first and second capacitors respectively coupled to the ends of the common coupling segment, and first and second transmission line segments directly connected to the respective resonant lines. In this manner, the resonator lines are coupled together at points of contact, rather than in a distributed capacitive manner between the lengths of the resonators.